1. Field of the Invention
The present invention relates in general to the improvement in a power factor of an electronic ballast for fluorescent lamps, and more particularly to a power-factor correction circuit of an electronic ballast for fluorescent lamps, in which a power transformer of a resonant inverter is coupled with charge pumping capacitors to improve a power factor of the ballast and automatically control power supply according to the number of fluorescent lamps being turned on.
2. Description of the Prior Art
Generally, electronic ballasts for fluorescent lamps have been recommended to satisfy the international standards such as IEC6100-3-2, which recommendation is recently on a trend of being changed to an obligatory rule. According to such a trend, there is a need for the development of techniques capable of meeting requirements such as a restriction in higher harmonic components of input current to an electronic ballast, an improvement in a power factor of the ballast, etc.
Well-known power-factor correction systems or filtering systems may generally be classified into a passive type and an active type. The passive systems mostly employ a structure using a low pass filter composed of only an inductor and a capacitor and a valley-fill structure. The low pass filter structure is advantageous in terms of cost, but disadvantageous in that apparent power being required is very large as compared with effective power, a longer harmonic distortion occurs and a rectified voltage has a great fluctuation with a load. Because of these disadvantages, the low pass filter structure is not so well used for systems requiring a high power-factor and stabilized power. On the other hand, the valley-fill structure is generally applied to circuits considering the volume and weight of an electronic ballast. However, the valley-fill structure has a disadvantage in that a direct current (DC) voltage waveform repeats a drop from a peak value to half that value, resulting in a flickering at 120 Hz under a high-frequency lighting condition. Such a flickering makes characteristics of discharge lamps instable, leading to a degradation in lighting efficiency.
Consequently, because power-factor correction circuits cannot satisfy a variety of requirements in the case of employing the above passive systems, they mostly utilize an active system employing a boost converter.
An active power-factor correction circuit employing the above boost converter is advantageous in that a DC-link voltage has a low ripple component and a good rectification characteristic and a flickering phenomenon is minimal, but has a disadvantage in that it is so considerably complicated in construction as to increase the cost of the overall product.
On the other hand, the performance of inverters is the kernel of electronic ballasts for discharge lamps. Such inverters may generally be classified into a voltage source type and a current source type. Among these inverters, a parallel resonant inverter of the current source type has been employed in electronic ballasts for fluorescent lamps being widely used, in consideration of a low-voltage driving function, a function of driving a plurality of fluorescent lamps, a variation in resonance characteristic with a load variation, etc.
FIG. 1a is a circuit diagram schematically showing the construction of a conventional power-factor correction circuit employing a boost converter system.
In the boost converter system of FIG. 1a, if a transistor Q is turned on, then the amount of current flowing to an inductor L.sub.boost is increased, thereby causing energy to be accumulated on the inductor L.sub.boost. Thereafter, when the transistor Q is turned off, the energy accumulated on the inductor L.sub.boost is transferred to an output stage through a freewheeling diode D. At this time, if the transistor Q is turned on before the amount of current through the freewheeling diode D becomes zero, a large amount of current flows from the diode D to the transistor Q for a period of reverse recovery time of the diode D, and it may break down the transistor Q.
The above problem can be overcome by controlling the amount of current I.sub.L through the inductor L.sub.boost in a discontinuous mode as shown in FIG. 1b by switching the transistor Q at the time that the amount of current through the freewheeling diode D becomes zero. However, in order to implement the above control operation, there is a need for a drive circuit for the transistor Q having a considerably complicated construction. Further, voltage and current stresses on devices may be increased and ratings of the devices may thus be raised, resulting in an increase in the cost of a product. This degrades the price competitiveness of the product.
On the other hand, in order to solve the above-mentioned degradation in the price competitiveness of the product resulting from the addition of the control circuit and the increase in the device ratings, there has been proposed a power-factor correction circuit as shown in FIG. 2a.
FIG. 2a is a circuit diagram showing the construction of a conventional low-price, electronic ballast employing the boost converter system.
With reference to FIG. 2a, the conventional electronic ballast can implement the power-factor improvement in the same manner as the above-mentioned power-factor correction circuit employing the boost converter system, by using only an inductor, diodes and a transformer without an additional switching control device [see: Marcio A. Co, J. L. Freitas Vieira, et al., IEEE PESC Transactions, pp. 962-968, 1996].
In more detail, in FIG. 2a, an inverter for driving fluorescent lamps includes two switches Q1 and Q2 which are driven in a self-excited manner. The switches Q1 and Q2 are alternately switched to generate square-wave voltage pulses, which are then applied to a resonance circuit through a transformer Tx1. In response to the square-wave voltage pulses generated by the switches Q1 and Q2, the resonance circuit generates a resonance voltage and resonance current of predetermined values at a high frequency and applies them to the fluorescent lamps. A power-factor correction circuit is connected between a set of rectifying diodes D1-D4 and a DC-link capacitor Cdc. The power-factor correction circuit includes a tertiary winding n3 of the transformer Tx1 provided for application of the square-wave voltage to the resonance circuit, and an inductor Lb and full-wave rectification diode circuit coupled with the tertiary winding n3 of the transformer Tx1, which has double the number of turns of a primary winding n1 of the transformer Tx1. The tertiary winding n3 of the transformer Tx1 generates a square-wave voltage corresponding to a predetermined turn ratio (n1:n3=1:2) as the switches Q1 and Q2 are switched. For one cycle of the square-wave voltage generated by the tertiary winding n3, a full-wave rectified version of an input voltage Vsrc from an alternating current (AC) input power source is applied to the inductor Lb and the corresponding current thus flows thereto, resulting in the formation of input current to the inverter.
The circuitry of FIG. 2a as mentioned above has a great effect in curtailing the cost because it is much simpler in construction than a conventional one comprising a separate boost converter. However, the above circuitry has a disadvantage in that a small and light capacitor cannot be replaced for the inductor Lb. In other words, the inductor Lb is structurally essentially required since the above circuitry employs the principle replaced for the separate boost converter and a square-wave voltage is generated across the tertiary winding n3 of the transformer Tx1 according to the switching operation of the switches Q1 and Q2. The circuitry of FIG. 2a has a further disadvantage in that the power-factor correction circuit cannot recognize a load connection state. This may cause a great variation in a DC-link voltage Vdc across the DC-link capacitor Cdc in the case where two fluorescent lamps are connected in parallel and one or both of them are selectively turned on.
FIG. 2b is a circuit diagram showing the construction of a conventional high power-factor electronic ballast employing a charge pumping capacitor, which improves a power factor using only the capacitor instead of an inductor on the basis of a charge pumping concept. This electronic ballast is disclosed in U.S. Patent, issued to Minoru Maechara, 1993.
With reference to FIG. 2b, the electronic ballast is provided with two main parts, or a power-factor correction circuit and a resonant inverter. The resonant inverter comprises a transformer Tx1 having a secondary winding connected to two fluorescent lamps connected in series, and preheating coils and capacitors connected respectively to filaments of the fluorescent lamps. The resonant inverter further comprises switches Q1 and Q2, a resonance circuit and a capacitor Cb for DC component prevention connected to a primary winding of the transformer Tx1. The resonance circuit is provided with a resonance inductor Lr and a resonance capacitor Cr. The switches Q1 and Q2 are controlled in a separate-excited manner to generate a resonance voltage, which is then applied to the fluorescent lamps through the secondary winding of the transformer Tx1 with an appropriate turn ratio.
The power-factor correction circuit has a simple construction consisting of only a charge pumping capacitor Cin and a diode Dc and performs the following operation. Assuming that a voltage Va across the resonance capacitor Cr is an individual high-frequency voltage source, the charge pumping capacitor Cin connected to the resonance capacitor Cr acts as a charge pump to allow the flow of current from an AC input power source to a DC-link capacitor Cdc through rectifying diodes D1-D4 and the diode Dc. In the case where a DC-link voltage Vdc across the DC-link capacitor Cdc is set to a value higher than a peak value of an input voltage Vsrc from the AC input power source by adjusting a capacitance of the charge pumping capacitor Cin, none of the diode Dc and rectifying diodes D1-D4 conduct, thereby causing the amount of charges being charged and discharged on/from the charge pumping capacitor Cin to vary in proportion to a variation of the input voltage Vsrc. As a result, because the average amount of input current traces the input voltage Vsrc, a power factor approximate to 1 can be obtained.
However, the above-mentioned power-factor correction circuit has a disadvantage in that a great variation may occur in the DC-link voltage Vdc as in the circuitry of FIG. 2a in the case where two fluorescent lamps are connected in parallel and one or both of them are selectively turned on. This power-factor correction circuit has a further disadvantage in that the charge pumping capacitor Cin exerts such an influence on the resonance operation according to a variation of the input voltage Vsrc as to generate a considerably high ripple component of 120 Hz in current flowing to the fluorescent lamps, resulting in an increase in crest factor (CF) of lamp current.
In order to overcome the above problems, there has been proposed a method for clamping the voltage Va to the DC-link voltage Vdc by adding diodes Da1 and Da2 to the structure of FIG. 2b as shown in FIG. 2c. However, this method encounters the occurrence of a conduction loss resulting from the production of a loop where current flowing to the resonance inductor Lr freewheels through the switches Q1 and Q2 and the diodes Da1 and Da2.